Ch09 Wide Area Networks Q&A

CHAPTER 9

WIDE AREA NETWORKS

Chapter Summary The Wide Area Network (WAN) is a key part of the enterprise edge. Most organizations do not

build their own WAN communication circuits, preferring instead to lease them from common

carriers or to use the Internet. This chapter focuses on the WAN architectures and

telecommunications services offered by common carriers for use in enterprise WANs, not the

underlying technology that the carriers use to provide them. We discuss the three principal types

of WAN services that are available: dedicated-circuit services, packet-switched services, and

virtual private network (VPN) services. We conclude by discussing how to improve WAN

performance and how to select services to build WANs.

Learning Objectives After reading this chapter, students should:

Understand dedicated-circuit services and architectures

Understand packet-switched services and architectures

Understand Internet-based VPN services and architectures

Understand the best practice recommendations for WAN design

Be familiar with how to improve WAN performance

Instructors Manual to Accompany: Business Data Communications and Networking, 12e by Fitzgerald/Dennis

2015 John Wiley & Sons, Inc. Page 1

Key Terms access VPN

Canadian Radio-Television

and

Telecommunications

Commission (CRTC)

channel service unit/data

service unit (CSU/DSU)

committed information rate

(CIR)

common carrier

dedicated-circuit services

discard eligible (DE)

Ethernet services

Encapsulating Security

Payload (ESP)

extranet VPN

Federal Communications

Commission (FCC)

fractional T1 (FT1)

frame relay

full-mesh architecture

interexchange carrier

(IXC)

Internet Service Provider

(ISP)

intranet VPN

IPSec

L2TP

latency

layer-2 VPN

layer-3 VPN

local exchange carrier

(LEC)

maximum allowable rate

(MAR)

mesh architecture

multiprotocol label

switching (MPLS)

packet

assembly/disassembly

(PAD)

packet services

Packet-switched services

partial-mesh architecture

permanent virtual circuits

(PVC)

point of presence (POP)

public utilities commission

(PUC)

ring architecture

star architecture

switched virtual circuit

(SVC)

synchronous digital

hierarchy (SDH)

synchronous optical

network (SONET)

T carrier circuit

T1, T2, T3, T4 circuits

virtual private network

(VPN)

VPN gateway

VPN software

Chapter Outline 1. Introduction 2. Dedicated-Circuit Networks

a. Basic Architecture b. T Carrier Services c. SONET Services

3. Packet-Switched Networks a. Basic Architecture b. Frame Relay Services c. Ethernet Services d. MPLS Services e. IP Services

4. Virtual Private Networks a. Basic Architecture b. VPN Types c. How VPNs Work

5. The Best Practice WAN Design 6. Improving WAN Performance

a. Improving Device Performance b. Improving Circuit Capacity c. Reducing Network Demand

7. Implications for Management



8. Summary

Answers to Textbook Exercises

Answers to End-of-Chapter Questions

1. What are common carriers, local exchange carriers, and interexchange carriers?

A common carrier is a private company that sells or leases communication services and

facilities to the public. Common carriers are profit-oriented, and their primary products are

services for voice and data transmissions, both over traditional wired circuits as well as cellular

services. Common carriers often supply a broad range of computer-based services, such as the

manufacturing and marketing of specialized communication hardware and software. Common

carriers that provide local telephone services are commonly called local exchange carriers

(LEC), while carriers that provide long distance services (e.g., Sprint) are commonly called

interexchange carriers (IXC). As the LECs move into the long distance market and IXCs move

into the local telephone market, this distinction may disappear.

2. Who regulates common carriers and how is it done?

Most countries have a federal government agency that regulates data and voice

communications. In the United States, the agency is the Federal Communications Commission

(FCC); in Canada it is the Canadian Radio-Television and Telecommunications Commission

(CRTC). The FCC regulates interstate and international communications to and from the United

States. State Public Utilities Commissions (PUCs) regulate intrastate communications within

their states. Regulation is achieved on the basis of tariffs filed and approved (or disapproved) by

the FCC and PUCs.

Most countries have a federal government agency that regulates data and voice

communications. In the United States, the agency is the Federal Communications Commission

(FCC); in Canada it is the Canadian Radio-Television and Telecommunications Commission

(CRTC). Each state or province also has its own public utilities commission (PUC) to regulate

communications within its borders.

The FCC/CRTC differs from the state PUCs in the following way:

The FCC has regulatory powers to compel common carriers to conform to the Federal Communications Act of 1934 and its revisions. It regulates the tariffs for interLATA and

international (calls to and from the United States) communications. These usually are

referred to as long distance communications.

A state PUC sets the rules and regulates the tariffs for all communications in its individual state boundary.

Note to the instructor: There may be some overlap between federal and state jurisdictions

because you can have both intraLATA and interLATA communications take place entirely

within one state.



3. How does MPLS work?

It is sometimes called a layer 2.5 technology because it inserts four-byte header that contains its own

information between the layer 2 frame and the layer 3 IP packet. With MPLS, the customer connects

to the common carriers network using any common layer 2 service (e.g., T carrier, SONET, ATM, frame relay, Ethernet). The carriers switch at the network entry point examines the incoming frame and converts the incoming layer 2 or layer 3 address into an MPLS address label. This label and some

other control information (e.g., quality of service (QoS)) form the MPLS header, which is inserted

into the layer 2 frame for transmission inside the carriers network.

4. Compare and contrast dedicated-circuit services, and packet-switched services.

With dedicated circuit networks, a circuit is established between the two communicating

computers. This circuit provides a guaranteed data transmission capability that was available for

use by only those two computers and is assigned solely to that transmission. No other

transmission is possible until the circuit is closed. In contrast, packet switched services enable

multiple connections to exist simultaneously between computers over the same physical circuit

or even over different physical circuits.

With a dedicated circuit network, you lease circuits from common carriers for their exclusive

use twenty-four hours per day, seven days per week. All connections are point to point, from

one building in one city to another building in the same or a different city. The carrier installs

the circuit connections at the two end points of the circuit and makes the connection between

them. The circuits still run through the common carrier's cloud, but the network behaves as if

you have your own physical circuits running from one point to another:

Dedicated circuits are billed at a flat fee per month and the user has unlimited use of the circuit.

Once you sign a contract, making changes can be expensive because it means rewiring the buildings and signing a new contract with the carrier. Therefore, dedicated circuits require

more care in network design than switched circuits both in terms of locations and the

amount of capacity you purchase.

With packet switched services, the user again buys a connection into the common carrier cloud).

The user pays a fixed fee for the connection into the network (depending upon the type and

capacity of the service) and is charged for the number of packets transmitted.

5. Is a WAN that uses dedicated circuits easier or harder to design than one that uses packet-switched circuits? Explain.

A WAN using dedicated circuits is harder to design than one that uses packet-switched circuits.

With dedicated circuits, once you sign a contract, making changes can be expensive because it

means rewiring the buildings and signing a new contract with the carrier. Dedicated circuits

therefore require more care in network design than using packet-switched circuits. In addition,

packet-switched networks enable packets from separate messages with different destinations to

be interleaved for transmission, unlike dedicated circuits.

6. Compare and contrast ring architecture, star architecture, and mesh architecture.



A ring architecture connects all computers in a closed loop, with each computer linked to the

next. The circuits are full duplex circuits. Computers in the ring may send data in one direction

or the other depending upon which direction is the shortest to the destination.

Properties of the ring architecture (assuming a double ring structure):

Messages can take a long time to travel from the sender to the receiver. Considering there are only two routes from any one computer to another, if one part of the circuit or any one

computer becomes overloaded, traffic delays can build up very quickly.

The double-ring structure offers redundancy. If a circuit fails, messages can travel to their destinations in the opposite direction (probably with a time and distance penalty). If the

network is operating close to its capacity, this will dramatically increase transmission times

because the traffic on the remaining part of the network may come close to doubling

(because all traffic originally routed in the direction of the failed link will now be routed in

the opposite direction through the longest way around the ring).

A star architecture connects all computers to one central computer that routes messages to the

appropriate computer.

Properties of the star architecture:

It is easy to manage because the central computer receives and routes all messages in the network.

It can be faster than the ring network since, in general any message needs to travel through fewer circuits to reach its destination than is the case in the ring network). However, the star

topology is the most susceptible to traffic problems because the central computer must

process all messages on the network. The central computer must have sufficient capacity to

handle traffic peaks or it may become overloaded and network performance will suffer.

In general, the failure of any one circuit or computer affects only the one computer on that circuit.

If the central computer fails, the entire network fails because all traffic must flow through it. It is critical that the central computer be extremely reliable.

In a mesh architecture (usually implemented as partial mesh architecture), every computer may

be connected to any other computer. Typically many, but not all, computers are connected.

Properties of the mesh architecture:

The effects of the loss of computers or circuits in a mesh network depend entirely upon the circuits available in the network. If there are many possible routes through the network, the loss

of one or even several circuits or computers may have few effects beyond the specific

computers involved. However, if there are only few circuits in the network, the loss of even one

circuit or computer may seriously impair the network.



In general, mesh networks combine the performance benefits of both ring networks and star networks. Mesh networks usually provide relative short routes through the network (compared

to ring networks) and provide many possible routes through the network to prevent any one

circuit or computer from becoming overloaded when there is a lot of traffic (compared to star

networks in which all traffic goes through one computer).

Since mesh networks use decentralized routing, each computer in the network performs its own routing. This requires more processing by each computer in the network than in star or ring

networks as well as the "overhead" transmission of network status information (e.g., how busy

each computer is), reducing network capacity.

7. What are the most commonly used T carrier services? What data rates do they provide?

T-Carrier Designation DS Designation Speed

DS-0 64 Kbps

T-1 DS-1 1.544 Mbps

T-2 DS-2 6.312 Mbps

T-3 DS-3 33.375 Mbps

T-3 DS-4 274.176Mbps

8. Distinguish among T-1, T-2, T-3, and T-4 circuits.

A T-1 circuit (sometimes called a DS-1 circuit) provides a data rate of 1.544 Mbps. T-1 circuits

can be used to transmit data, but often are used to transmit both data and voice. In this case, a

time division multiplexer (TDM) provides 24 64 Kbps circuits. Digitized voice using pulse code

modulation (PCM) requires a 64 Kbps circuit so a T-1 circuit enables 24 simultaneous voice

channels.

A T-2 circuit transmits data at a rate of 6.312 Mbps. Basically, it is a multiplexed bundle of four

T-1 circuits. A T-3 circuit allows transmission at a rate of 44.376 Mbps, although most articles

refer to this rate as 45 megabits per second. This is equal to the capacity of 28 T-1 circuits. T-3

circuits are becoming popular as the transmission medium for corporate WANs because of their

higher data rates. At low speed, these T-3 circuits can be used as 672 different 64 Kbps channels

or voice channels. A T-4 circuit transmits at 274.176 Mbps, which is equal to the capacity of

178 T-1 circuits. Obviously, an organization using either T-3 or T-4 circuits must have a

tremendous need to transmit very large quantities of data.

9. Describe SONET. How does it differ from SDH?

The synchronous optical network (SONET) has recently been accepted by the U.S. standards

agency (ANSI), as a standard for optical transmission at gigabit per second speeds. The

international telecommunications standards agency (ITU-T) also recently standardized a version

of SONET under the name of synchronous digital hierarchy (SDH). SONET and SDH are very

similar and can be easily interconnected. The SONET standard includes more data rates than the

SDH standard. As with T carrier services, a CSU/DSU is needed to connect the user's network

into the SONET/SDH circuit.



SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of 51.84 Mbps.

Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1, with

SONET data rates defined as high as OC-192 or about 10 Gbps. Each level above OC-1 is

created by multiplexing. Although not yet available in all locations, SONET/SDH is available in

most large cities worldwide.

10. How do packet-switching services differ from other wide area networks services?

Packet switched services are quite different from the other types of network services. For each

of these three, dialed circuit services, dedicated circuit services, and circuit switched services, a

physical circuit was established between the two communicating computers. This circuit

provided a guaranteed data transmission capability that was available for use by only those two

computers.

In contrast, packet switched services enable multiple connections to exist simultaneously

between computers. With packet switched services, the user again buys a connection into the

common carrier network. The user pays a fixed fee for the connection into the network and

charged for the number of packets transmitted.

11. Where does packetizing take place?

Splitting messages into individual packets (packetizing) takes place at a packet

assembly/disassembly device (PAD), which can be owned and operated by the customer or by

the common carrier.

The PAD converts the sender's data into the network layer and data link layer packets used by

the packet network and sends them through the packet switched network. At the other end,

another PAD reassembles the packets back into the network layer and data link layer protocols

expected by the destination and delivers it to the appropriate computer. This "packetizing" and

re-assembly is almost instantaneous, and data are transmitted continuously. The PAD can

translate between different data link layer protocols between the sender and the destination (e.g.,

ethernet at the sender and token ring at the receiver). It may also provide conversion from one

code to another (i.e., ASCII to EBCDIC).

12. Compare and contrast frame relay, MPLS, SMDS, and Ethernet services.

Frame relay differs from traditional networks in three important ways. First, frame relay

operates only at the data link layer. Frame relay, like other packet switched networks takes the

incoming packets from the user network and converts them to its own packet structure for

internal transmission. Frame relay uses variable length packets which adapt to the size of the

incoming packet (up to 8K).

Second, frame relay networks do not perform error control. Virtually all other types of networks

perform error checking at each computer in the network. Any errors in transmission are

corrected immediately, so that the network layer and application software can assume error-free

transmission. However, this error control is one of the most time consuming processes in a

network. Most networks today are relatively error-free, so frame relay networks do not ensure



error-free delivery of the packets (they do perform error checking, but simply discard packets

with errors; they do not generate NAKs and ask for retransmission). It is up to the software at

the source and destination to perform error correction and to control for lost messages. Since the

user's data link packet remains intact, it is simple for the devices at the edge of the frame relay

network to check the error control information in the user's data link layer packet to ensure that

no errors have occurred and to request transmission of damaged or lost packets.

A third major difference is that frame relay defines two connection data rates that are negotiated

per connection and for each virtual circuit as it is established. The committed information rate

(CIR) is the data rate the circuit must guarantee to transmit. If the network accepts the

connection, it guarantees to provide that level of service. Most connections also specify a

maximum allowable rate (MAR), which is the maximum rate that the frame relay network will

attempt to provide, over and above the CIR. The circuit will attempt to transmit all packets up to

the MAR, but all packets that exceed the CIR are marked as discard eligible (DE). If the

network becomes overloaded, DE packets are discarded. So while can transmit faster than the

CIR, they do so at a risk of lost packets.

Switched Multimegabit Data Service (SMDS) is an unreliable packet service like ATM and

frame relay. SMDS encapsulates incoming packets from the user's network with ATM-like 53-

byte cells, although the address is different than an ATM address. The user's data link layer

address is mapped to the SMDS address, which is used for transmission through the SMDS

network. The SMDS cell is stripped off at the destination and the user's data link layer packet

reassembled. Like ATM and frame relay, SMDS does not perform error checking; the user is

responsible for error checking. SMDS provides only a connectionless datagram service.

Ethernet service networks bypass the PSTN; companies offering Ethernet/IP packet networks

have laid their own gigabit Ethernet fiber optic networks in large cities. When an organization

signs up for service, the packet network company installs new fiber optic cables from their city-

wide WAN backbone into the organization's office complex and connect it to an Ethernet

switch. The organization simply plus their network into their Ethernet switch and begins using

the service. All traffic entering the packet network must be Ethernet using IP. Since most

organizations today use Ethernet and IP in the LAN and BN environment, Ethernet/IP avoids

the need to translate or encapsulate to generate addresses for LAN or BN traffic and gains in

throughput. It avoids complexity, meaning that companies do not have to add staff

knowledgeable in the different WAN protocols, software, and hardware these technologies

require. This technology is offered by relatively new startup companies like Yipes.com.

MPLS is different in that it is designed to work with a variety of commonly used layer-2

protocols. The customer connects to the common carriers network using any common layer-2 service. MPLS offers a wide range of speeds because it can run on a variety of physical circuits

such as T-carrier and SONET.

13. Which likely to be the longer term winner, IP, MPLS, or Ethernet services?

Each of these technologies has benefits and an argument could be made for each. As such,

student answers will vary. IP seems to be a contender because the Internet utilizes TCP/IP.

Ethernet has the advantage of being used heavily on LANs, so the protocol conversion doesnt



have to occur. MPLS is a relatively new option and has the benefit of being able to work so

closely with Layer 2 and Layer 3 protocols.

14. Explain the differences between CIR and MAR.

The committed information rate (CIR) is the data rate the circuit must guarantee to transmit. If

the network accepts the connection, it guarantees to provide that level of service. Most

connections also specify a maximum allowable rate (MAR), which is the maximum rate that the

frame relay network will attempt to provide, over and above the CIR. The circuit will attempt to

transmit all packets up to the MAR, but all packets that exceed the CIR are marked as discard

eligible (DE). If the network becomes overloaded, DE packets are discarded. So while can

transmit faster than the CIR, they do so at a risk of lost packets.

15. How do VPN services differ from common carrier services?

A type of VAN, called a virtual private network (VPN) (or sometimes software defined

network) has emerged. VPNs provide circuits that run over the Internet but appear to the user to

be private networks. Internet access is inexpensive compared to the cost of leasing dedicated

circuits, circuit switched services, or packet switched services from a common carrier.

Different VPNs provide different services, but most offer packet switching hardware that will

communicate via the Internet, or VPN services which you lease from the VPN in much the same

way as leasing a service from a common carrier. These VPN hardware (or services) take your

data, encrypt it, and send it through the Internet through a series of tunnels -- a virtual circuit through the Internet which constrains the source and destination to only those within the VPN.

16. Explain how VPN services work.

A virtual private network (VPN) provides the equivalent of private packet switched network

over the public Internet. You establish a series of PVCs that run over the Internet, so that the

network acts like a set of dedicated circuits over a private packet network.

With a VPN, you first lease an Internet connection at whatever access rate and access

technology you choose for each location you want to connect. For example, you might lease a

T-1 circuit from a common carrier that runs from your office to your Internet service provider

(ISP). You pay the common carrier for the circuit and the ISP for Internet access. Then you

connect a VPN device (a specially designed router or switch) to each Internet access circuit to

provide access from your networks to the VPN. The VPN devices enable you to create PVCs

through the Internet that are called tunnels.

The VPN device at the sender takes the outgoing packet and encapsulates it with a protocol that

is used to move it through the tunnel to the VPN device on the other side (a technology focus

box later in this chapter describes this process in more detail). The VPN device at the receiver,

strips off the VPN packet and delivers the packet to the destination network. The VPN is

transparent to the users; it appears as though a traditional packet switched network PVC is in

used. The VPN is also transparent to the ISP and the Internet as a whole; there is a simply a

stream of Internet packets moving across the Internet.



The primary advantages of the VPNs is low cost and flexibility. Because they use the Internet to

carry messages, the major cost is Internet access, which is inexpensive compared to the cost of

circuit switched services, dedicated circuit services, or packet switched services from a common

carrier. Likewise, anywhere you can establish Internet service, you can quickly put a VPN.

There are two important disadvantages.

Since traffic on the Internet is unpredictable, sometimes packets travel quickly and at other times they take a long time to reach their destination.

Second, because the data travels on the Internet, security is always a concern. Most VPN networks encrypt the packet at the source VPN device before it enters the Internet and

decrypt the packet at the destination VPN device.

17. Compare the three types of VPN.

Three types of VPN are in common use: intranet VPN, extranet VPN and access VPN.

An intranet VPN provides virtual circuits between organization offices over the Internet. Each location has a VPN device that connects the location to another location through the

Internet.

An extranet VPN is the same as an intranet VPN except that the VPN connects several different organizations, often customers and suppliers, over the Internet.

An access VPN enables employees to access an organization's networks from a remote location. Employees have access to the network and all the resources on it in the same way

as employees physically located on network. The user connects to a local ISP that supports

the VPN service via POTS, ISDN, or other circuit. The VPN device at the ISP accepts the

user's login, establishes the tunnel to the VPN device at the organization's office, and begins

forwarding packets over the Internet. An access VPN provides a more less expensive

connection than having a national toll-free 800 number that connects directly into large sets

of modems at the organization's office. Compared to a typical ISP-based remote connection,

the access VPN is secure connection than simply sending packets over the Internet.

18. How can you improve WAN performance?

Improving the performance of WANs is handled in the same way as improving LAN

performance. You begin by checking the devices in the network, by upgrading the circuits

between the computers, and by changing the demand placed on the network. Below is the

performance checklist for improving WANs.

Increase Computer and Device Performance

Upgrade devices

Change to a more appropriate routing protocol (either static or dynamic)

Increase Circuit Capacity

Analyze message traffic and upgrade to faster circuits where needed

Check error rates



Reduce Network Demand

Change user behavior

Analyze network needs of all new systems

Move data closer to users

19. Describe five important factors in selecting WAN services.

Five important factors in selecting WAN services are vendor capabilities, network capacity,

flexibility, control, and reliability. The best vendors provide high quality service, quickly

respond to network problems, adapt to changing customer needs, and provide useful network

management services along with the data transmission services.

There are a variety of services available at many different data transmission rates. Try to

estimate the general capacity you need at each network site, and be aware that users needs change. In general, dedicated circuits are much less flexible than switched services. Control is

another important issue. With dedicated circuits, you have more control over how you messages

get routed in the network because your computers do the routing. With switched services, the

service provider is responsible for the routing, and, your messages get intermixed with those of

other network users. The reliability of a network service both in terms of average error rates and

any circuit failures is also important.

20. Are Ethernet services a major change in the future of networking or a technology blip?

It is reasonable to expect major changes in the future of networking with Ethernet services.

Several carriers have announced that they intend to stop offering all services except Ethernet

and Internet services. With the advent of Ethernet services such as YIPES, this offers a

significant approach to networking because it offers a way to "slice" fiber services down to the 1

Mbit level. Thus the customer can dynamically allocate slices and pay for what is used without

the costs of infrastructure change. Further, as organizational LANs heavily use Ethernet and IP

in the LAN and BN environment, and the WAN packet network services (X.25, ATM, Frame

Relay, and SMDS) use layer-2 protocols, it is likely that Ethernet services would be more

attractive. Any LAN or BN traffic must be translated or encapsulated into a new protocol and

destination addresses generated for the new protocol. This takes time, slowing network

throughput. Thus, the advantage of Ethernet services in networking is that there is no

translation prior to transmission, making the service appealing. Ethernet services will represent

a new and very attractive WAN technology in the future.

21. Are there any WAN technologies that you would avoid if you were building a network today? Explain.

FDDI because of bandwidth issues and high hardware costs, Integration of transport favors

technologies using IP addressing. At this point, the avoidance of ATM in newer networks is

suggested since so much new development is using Ethernet.

22. Suppose you joined a company that had a WAN composed of SONET, T carrier and frame relay services, each selected to match a specific network need for a certain set of circuits.

Would you say this was a well-designed network? Explain.



This depends on how suitable the various technologies are for the applications supports. Today,

increasingly, network designers make decisions to reduce costs, gain reliability, and increase

performance through network integration, dynamic allocation so that unused capacity much not

be paid for, and failover capabilities.

23. It is said that frame relay services and dedicated-circuit services are somewhat similar from the perspective of the network designer. Why?

They are both based on a single connection to the common carrier and provide similar

transmission speed and reliability.



Mini-Cases

I. Cookies Are Us

Cookies are Us runs a series of 100 cookie stores across the Midwestern U.S. and central

Canada. At the end of each day, the stores sends sales and inventory data to headquarters, which

uses the data to ship new inventory and plan marketing campaigns. They have decided to move

to a WAN. What type of a WAN architecture and WAN service would you recommend?

Based on limited application requirements, VPN or T-carrier services will work well. Each of

these are relatively cheap and will suffice for the file transfers required daily at the headquarters

site.

II. MegaCorp

MegaCorp is large manufacturing firm that operates 5 factories in Dallas, 4 factories in Los

Angeles, and 5 factories in Albany, New York. It operates a tightly connected order

management system that coordinates orders, raw materials, and inventory across all 14 factories.

What type of a WAN architecture and WAN service would you recommend?

Student answers can vary on this one based on some assumptions that can be made related to the

volume of data and current costs. T carrier services would be a good option, as these would

provide the necessary bandwidth at a reasonable price. .

III. Sunrise Consultancy

Sunrise Consultancy is a medium-sized consulting firm that operates 17 offices around the world

(Dallas, Chicago, New York, Atlanta, Miami, Seattle, Los Angeles, San Jose, Toronto, Montreal,

London, Paris, Sao Paulo, Singapore, Hong Kong, Sydney, and Bombay). They have been using

Internet connections to exchange email and files, but the volume of traffic has increased to the

point that they now want to connect the offices via a WAN. Volume is low but expected to grow

quickly once they implement a new knowledge management system. What type of a WAN

topology and WAN service would you recommend? Why?

With Sunrise moving from the Internet to a Wide Area Network (WAN), I would suggest a Cloud-

based design, as that will be the simplest method for the network manager to network in the nine

independent offices.

IV. Cleveland Transit

Reread Management Focus 9-1. What other alternatives do you think Cleveland Transit

considered? Why do you think they did what they did?

Cleveland Transit probably considered many of the packet-switched services. One of the main

reasons they chose a SONET ring was because of the full-duplex ring which enables

communication to continue even in the event of a circuit cut. Resiliency of the network is key due

to the 24/7 requirement for the types of services that they require.



V. Air China

Reread Management Focus 9-2. What other alternatives do you think Air China considered?

Why do you think they did what they did?

They probably considered many of the packet-switched services as they offer some of the same

benefits. The selection of Frame Relay is not surprising because it is one of the more common

world-wide and operates at sufficient speeds for this type of application.

VI. Marietta City Schools

Reread Management Focus 9-3. What alternatives do you think Marietta City Schools

considered? Why do you think they did what they did?


benefits. They probably selected Ethernet because they probably are using Ethernet on their

LANs so no protocol translation is required. In addition, the 1-10 Gbps speeds offer the service

they need for the new application types they are considering.

VII. Cisco

Reread Management Focus 9-4. What other alternatives do you think that Cisco considered?

Why do you think they did what they did?


benefits. They probably selected the MPLS option because they were able to utilize this option to

provide a full mesh architecture to increase reliability, while at the same time increasing

capability and flexibility, while keeping costs about the same.

Next Day Air Service Case Study

1. With your knowledge of NDAS's network, what service would you recommend for the

future to connect the remote offices to the hubs at Atlanta and New Orleans and the hubs to the

corporate office in Tampa? Will the current facilities be adequate?

Based on the traffic volume analyses done for this case study in earlier chapters, the existing

facilities seem adequate for reasonable workload increases in the existing offices. As Next Day

Air Service grows into a nationwide operation, however, the multiplexed circuits between

Tampa/Atlanta and Tampa/New Orleans will have to be replaced. Additional offices can be

handled by adding another circuit between the new office and the appropriate multiplexer hub in

Atlanta or New Orleans. Also, more multiplexers might be added, as was discussed previously.

The current trend is to combine both voice and data communications onto a single network. To

combine voice and data, the students might recommend acquisitions of an ISDN facility with

basic 2B + D channel service linking each remote office to the appropriate hub. Each hub should

be connected to the Tampa office using primary 23B + D channel service

Another alternative might be the acquisition of Megacom or WATS circuits, This would require

incoming and outgoing Megacom circuits for interstate calling, as well as incoming and outgoing

Megacom circuits for interstate calling.

Answers may vary. Encourage student discussion of other digital services, such as T-1 circuits,

or packet switched services. Accept any reasonably justified alternative answer.

2. President Coone has just informed you that NDAS is considering placing several new

offices in Chicago and Los Angeles. Each office would have its own LAN. What factors would



determine the use of a metropolitan area network (MAN) to connect NDAS offices in a single

city together?

The key factors in determining the use of a MAN to interconnect NDAS offices in a single city

are:

a. the availability of MAN services within the city b. the cost of the MAN service (using the common carrier or even construction of NDASs

own MAN)

c. the estimated volume of the data to be transmitted between and among the LANs linked via MAN service

d. the need for dial-in and dial-out service on the LANs e. the actual distances between LANs f. the nature and cost of interfacing software and equipment needed to access the MAN g. network management requirements for MAN integrated LANs h. network reliability considerations for MAN integrated LANs i. network security on MAN integrated LANs

The topic of MAN-linked LANs should lead to an interesting class discussion, especially if a

local Bell Operating Company representative, knowledgeable about MAN services offered in

your city, can be invited as class discussion moderator.

3. As the network plans become more global, what security issues become more of a concern?

The international nature of the expanded network creates a greater security concern for the

company. Many countries overseas do not have the same security and privacy laws as we seen in

the U.S. , making it much more difficult to capture and prosecute those attempting to breach

security. Similarly, some cultures abroad dont consider hacking, copyright infringement, etc to be such a bad thing. Thus, as operations expand into these countries, more potential attacks are

likely to occur.

Additional Content

Teaching Notes

I cover the material in this chapter evenly, but I omit the selecting WAN services. My primary

goal is to have students understand the major differences between the general categories of

services and to have some familiarity with the specific services offered. In many ways, dialed

services and circuit switched services are the same and I point this out to students. Likewise, the

data rates for many services are similar so I point out that most of the internal network

plumbing uses 64 Kbps PCM circuits and this is why the data rates are similar. The primary differences are the connection features (dedicated, circuit switched, packet switched) and the

way in which the services are marketed to the users.

Dedicated circuit services have remained mostly stable of the past few years, with the exception

of ever increasing data rates. The only new entrant is DSL and its derivatives (e.g., ADSL,

VDSL) and competitors (cable modems). This is an area to watch. There will be many new

developments over the next few years as this technology matures.



Circuit switched technologies (i.e., ISDN) have now entered the main stream. Im not sure ISDN will be that important over the next three years. If the RBOCs deploy ADSL as rapidly as some

are planning to, then ADSL may eclipse ISDN. I read an article in a telecommunications

magazine entitled ISDN: We Hardly Knew You. There may be some truth in this.

War Stories

Installing ISDN

(Objective: illustrate the problems with ISDN)

A friend of mine who worked for AT&T attended a 2-day seminar on installing ISDN in New

York. The entire first day (except for some introductory remarks) was spent watching the

instructor call six different organizations to get provisioning information needed to connect the

ISDN card to the telephone network and have it talk to the training organizations server over the ISDN line. Im not joking. After six hours worth of phone calls and several incorrect responses to the instructors questions requiring additional calls, the computer was finally able to talk with the organizations server. With stories like this, it is no wonder that organizations are looking toward ADSL rather than ISDN.

Ch09 Wide Area Networks Q&A

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Transcript of Ch09 Wide Area Networks Q&A